Need help changing an irrational number into a rational number, involving polar coord

I am working with problems of changing from Polar to Cartesian coordinates.

the problem is (-5, pi/3)
I know x=-5cos pi/3 = -5/2
and y = -5sin pi/3 \approx 4.33 or = 5sqrt3/2 but I only know that it can be written as 5sqrt3/2 because after I got 4.33 I check the answer. I need to know how to convert numbers like 4.330127019 in 5sqrt3/2